Like any other machine or its part, propellers of ships need maintenance from time to time, replacement of parts thereof as the case may be, or simply need repair when damaged for whatever reason.
There are basically two types of propellers, the first type is a usually single piece propeller with fixed blades, whereas the other usual type is an adjustable pitch propeller in which the blades are arranged so as to be adjustable with respect to a propeller hub.
There are usually different ways to carry out maintenance and/or repair of the propellers, these methods comprising docking a ship, so that the ship is outside the water and access to the propeller is easily possible also with large equipment.
Another possibility is to use divers who carry out repair work under water. However, in case of adjustable pitch propellers, there is the problem that removal of the blade from the hub necessarily exposes a hub opening to the seawater, so that seawater can enter the hub mechanism and/or oil is released form the hub mechanism. However, any release of oil into the water has to be strictly avoided because oil released from the hub causes water pollution, and seawater entering the hub mechanics may cause corrosion and later destroy the hub mechanics.
On the other hand, in the course of ordinary maintenance, sealings which are arranged at the root of the blade to seal against the hub for closing the hub opening, have to be replaced from time to time, since there occurs a certain wear when the blades are adjusted.
Document GB 22 55 582 A suggests a method and an apparatus for the maintenance of the propeller, i.e. exchange of a blade or of a blade sealing, using a housing which is placed or pulled over at least the uppermost blade and the hub. That housing is securely fixed to the hub and is then filled with compressed gas so as to displace the water from the housing in order to provide a dry space around the blade foot at the hub, so that persons entering the housing can work inside in a dry space. The mentioned document either suggests a huge solid housing with an extension to cover at least part of the propeller shaft or suggests a flexible housing which covers the blade and the hub.
However, the solid housing proposed in GB 22 55 582 A is a large and heavy element which requires adequate equipment for handling. Since ship propellers are quite large elements, the flexible housing covering at least the diameter of the ship propeller, as is mentioned in this document, is also large. Already due to the size, handling of these housings is complicated.